The present invention relates to improvements in pump and motor units, especially in hermetically sealed pump and motor units wherein an electric motor (preferably a wet motor) drives a centrifugal pump for circulation of water or another hydraulic fluid in a boiler or the like. More particularly, the invention relates to improvements in thrust bearing assembles for axially movable, axially stressed motor shafts in pump and motor units.
Boiler circulators in the form of pump and motor units must frequently operate at extremely high system pressures and temperatures. Such units must remain effective during each and every stage of operation including one or more stages during which the temperature and/or pressure of circulated fluid fluctuates within an extremely wide range. Many recent types of boilers employ at least one but normally two pairs of pump and motor units which are connected in parallel. When the boiler is started, one of the units normally operates under overload prior to starting of the other unit or units. At such stage of boiler operation, the operating point of the one unit is well within the overload range so that axial stresses upon the motor shaft which carries the impeller of the pump are extremely high due to suction which develops in the region of the impeller and causes a pronounced rise in axial and radial stresses.
The situation is analogous during normal operation of the boiler, i.e., when the liquid supplying pump begins to feed liquid into the boiler. The one recirculating pump and motor unit is then caused to coast (the operating point then lies in the fourth force quadrant of the flow-head performance curve, i.e., in the negative range) which also results in generation of pronounced stresses upon the bearings for the motor shaft. In order to save energy, the operators prefer to disconnect the motor of the circulating pump and motor unit from the source of electrical energy which results in an operation known as windmilling. The unit then constitutes an additional resistance in the system and its rotary parts can turn at speeds ranging from zero to well beyond the normal operating speed. When the rotational speed is low, the customary hydrodynamic bearings are incapable of resisting the developing axial stresses. Therefore, the construction of thrust bearings in such units is of great importance and the presently known bearings are often incapable of taking up stresses during each and every stage of boiler operation.